Amongst electrode plates for rechargeable batteries, those produced using a foamed metal with a three dimensional network structure as a core substrate, by impregnating the core substrate with an active material, display comparatively superior discharge capacities, and are widely used. In addition, in recent years there has been strong demand for improvements in the high rate discharge characteristics of batteries, and as a result, new battery electrode plate manufacturing methods have been proposed, such as that shown in FIG. 7A to FIG. 7E disclosed in Japanese Laid-Open Patent Publication No. 2000-77054. Firstly, in a first pressing process, two slots 2 of predetermined width are formed in a core substrate 1 composed of a foamed metal, with the two slots parallel with both edges of the core substrate. Once the core substrate 1 has been impregnated with an active material 3, the active material 3 accumulated inside the slots 2 is removed using a brush or the like. Subsequently, in a second pressing process, the core substrate 1 is subjected to three press working steps and converted to a form shown in FIG. 7C in which the entire surface is level with the bottom of the slots 2. The sections where the slots 2 had been formed are then subjected to an active material removal process using a brush and an air blower to form core substrate exposed sections 4 as shown in FIG. 7D. The core substrate 1 is then cut, forming battery electrode plates 7.
A current collector 7b including the core substrate exposed section 4 is formed on one edge of the battery electrode plate 7, and a cylindrical electrode group formed by winding this electrode plate has a current collector on one end surface. Because this electrode group collects current uniformly along the entire length of the battery electrode plate, the current collecting efficiency improves. In addition, by employing a tab-less method wherein a current collecting lead plate is welded to the aforementioned current collector, the current collection characteristics improve markedly, enabling the demands for improvements in high rate discharge characteristics to be met.
However, the battery electrode plate 7 prepared by the processes described above suffers from the problems described below. A first problem is that because variations in the impregnation density of the active material 3 occur within active material impregnated sections 7a, there is a variation in the capacity of batteries produced using these battery electrode plates 7, and so when applied to a battery pack, there is an increased likelihood of over charging or over discharging.
A second problem is that because a boundary line 7c between the active material impregnated section 7a and the current collector 7b is not a true straight line, the precision of the dimensions and shape of the battery electrode plate 7 is low, leading to a reduction in the current collecting function of a battery produced using this battery electrode plate 7, and a failure to achieve high rate discharge characteristics.
A third problem is that because the removal of the active material 3 from the current collector 7b is imperfect, there is an increased likelihood of unsatisfactory welding occurring during attachment of the current collecting lead plate to the current collector 7b, resulting in a reduced yield. Removal of the active material using a brush and air blower is also inefficient, and invites a reduction in productivity.
A fourth problem is that the width of the core substrate exposed sections 4 shown in FIG. 7D, prior to cutting, differs from the preset value. As a result, a method wherein the core substrate exposed section is folded at right angles and then compressed to form the current collector cannot be applied, and so it becomes impossible to ensure the mechanical strength of the current collector or a high current collection efficiency.
A fifth problem is that the battery electrode plates 7 obtained by cutting the core substrate 1 are susceptible to warping into a bow shape. When the battery electrode plate 7 is wound into a spiral shape to form an electrode group, this warping can be the cause of weaving, resulting in an electrode group of an unsatisfactory shape. Moreover, not only does this warping occur, but when viewed at magnification under a microscope, it is apparent that fine cracks also develop at the boundary section between the active material impregnated section 7a and the current collector 7b, and sections of the metallic skeleton of the core substrate 1 rupture, leading to a deterioration in strength. As a result, this type of battery electrode plate 7 is susceptible to problems such as dropout of the active material 3, short circuiting, and deterioration in the electrical conductivity.
Japanese Laid-Open Patent Publication No. 2000-77054 discloses another method of manufacturing a battery electrode plate. This method involves impregnating an entire core substrate composed of a foamed metal with an active material, subsequently carrying out press working to compress the entire core substrate to a predetermined thickness, and then forming core substrate exposed sections by removing the active material from certain regions using an ultrasonic vibration device.
However in this method, because the boundary line between the active material impregnated sections and the current collector of the battery electrode plate is not a true straight line, there is a deterioration in the current collecting function of a battery produced using this battery electrode plate, and high rate discharge characteristics are unobtainable. This is because a large amplitude ultrasonic vibration must be applied in order to remove the active material after the press working, and as a result, even the active material in the regions surrounding the core substrate exposed sections is removed. In addition, there is a danger that the metallic skeleton of the core substrate may suffer damage or deterioration when exposed to large amplitude ultrasonic vibrations.
Consequently, the present invention takes the conventional problems described above into consideration, with an object of providing a method and apparatus for manufacturing a battery electrode plate in which there is no variation in the impregnation density of the active material, the boundary line between the active material impregnated sections and the current collector is a true straight line, the residual ratio of the active material in the current collector is low, and the entire current collector has a predetermined width, as well as providing a battery which utilizes such a battery electrode plate.